The present invention relates to blind rivets, and more particularly, but not exclusively, to a blind rivet with high shear resistance.
A blind rivet assembly normally comprises a rivet body and a mandrel assembly. The rivet body comprises a flange portion and a cylindrical body that extends from the flange portion and an axial hollow bore that passes through the rivet body. A mandrel typically comprises a head portion and a long stem that passes through the rivet body, such that the mandrel head portion abuts a blind-side end of the rivet body.
In use, the rivet assembly is inserted into a prepared hole in a workpiece or application, so that the flange portion of the rivet body abuts an operator's side of the application. A workpiece or application typically consists of two, or more, sheets of material. In order to achieve setting a setting tool is attached to the blind rivet by sliding a nosepiece of the setting tool over the mandrel stem, such that the nosepiece of the setting tool abuts the flange of the rivet body.
On operation, jaws of the setting tool grip the mandrel stem and retract, such that a mandrel head portion applies an axial compressive load to one end of the rivet body. As the setting load increases, the rivet body experiences compressive loading. The compressive loading pulls the application parts together. The rivet body then deforms to fill the hole in the application. As the setting load increases further, the application parts are secured and the mandrel breaks at a predetermined breaking load. The setting is then complete.
The set rivet thus comprises a rivet body with its flange firmly abutted against the work piece; a radially expanded body on the blind-side and the residual mandrel head portion that remains in the rivet body. It can be seen that the mandrel is instrumental in providing the setting function in applying the rivet but it does not, however, contribute to the structural rigidity of the set rivet. For instance if the rivet were to be subjected to shear loads, for example two opposing forces in each sheet of the application at an angle to the longitudinal axis of the rivet, the rivet body will tend to tilt with its flange edge digging into one side of the application and the set-end into the opposite or blind side. Since the rivet is formed from a relatively softer material than the application, it tends to bend. If loading is cyclical there is a risk that the rivet body ultimately fails from fatigue induced stresses. In such instances a long break mandrel is usually specified. However, since such a long break mandrel also tilts, when subjected to shear forces, with the rivet body, (and as a result of its relatively smaller diameter), it does not contribute to the structural integrity or strength of the joint. This is a particularly acute problem where there is clearance between the mandrel shank diameter and the internal bore of the set rivet, as such a gap offers no resistive strength to a turning moment. This can be seen from published European Patent Application EP-A-0 945 631.
There are some assemblies, such as seats for passengers and drivers in automobiles, which are required to be securely anchored to an automobile frame and to be of rigid construction in order to resist impact and dynamic forces, that may be generated for example, during an accident.
A common method of seat construction uses blind rivets since they are low cost, easy to apply and contribute to the high productivity rates that are required by the automotive industry. Furthermore blind rivets are ideal where access to the blind-side or side remote from the assembly operator is difficult. This provides design flexibility and enables high production rates through semi or fully automated assembly.
With the ever increasing desire to improve rigidity and safety of seating construction, blind rivets or fasteners are required to be increasingly stronger, in particular in their shear resistance. One such method for improved strength is to make the rivet or fastener greater in diameter, as a greater amount of material provides improved strength and shear resistance and provides a greater bearing area to support applied loading.
The present invention seeks to provide an improved blind rivet which has improved strength and shear resistance.
A blind rivet body usually has a relatively thick wall section and is made from a relatively soft material, as the soft material is conducive to filling the hole in the application and forming a blind-side bulge on setting. Thick wall cylinders, however, formed from a relatively soft material such as a low-carbon steel, do not offer the optimum resistance to high shear stresses resulting from impact loads.
The present invention further seeks to provide a rivet with an improved shear resistance despite employing soft deformable materials in a blind rivet.